"Bind off 2 sts at beg of next 4 rows, then dec 1 st at each side [every other row] 3 (3,4,4,5) times, (36, 38,40,46,48 sts)
Continue to work even in St st until armholes measure approx 3 (3, 3 1/2, 4 1/4, 4 3/4) inches from beg, ending with a WS row."

Mainly what I am not clear is on the underlined part. But just for clarification sake and making sure I did the whole part correctly, can some explain it in a bit more detail?